The present invention relates to the magnet arts. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with spectroscopy, magnetic therapy, and the like.
Many magnetic resonance imaging systems have a patient receiving bore surrounded by a series of annular magnet windings. In an annular bore system, access to the patient is limited. Moreover, such systems are claustrophobic to some patients.
To obtain more openness, magnetic resonance imaging systems have been created in which one annular magnet is disposed above a patient gap and another annular magnet is positioned below the patient gap. The patient is horizontally received between the magnets. Typically, ferrous pole pieces are connected with the magnets to improve the linearity and strength of the vertical field. Commonly, the pole pieces are connected with a C-shaped iron yoke which defines a flux return path between the two annular driver coils situated on either side of the patient gap. In order to accommodate the relatively high magnetic fluxes flowing through the C-arm without saturating, the C-arm has a substantial cross-section. The C-shaped iron yoke also supports the magnets and the pole pieces. The C-shaped iron yoke has sufficient strength to resist the 50-100 tons of attractive force between the pole pieces.
For a 0.5 T magnet with a 65 cm gap, a C-arm with adequate strength and magnetic flux capability weights about 40 tons. Such an arm is too heavy for normal transportation in one piece. It is also not practical to assemble, shim, and test such magnets at the customer site.
To accommodate these concerns, magnets have been constructed with two or even four symmetric, iron flux return paths. However, the multiple return paths provide multiple interferences with access to the patient. Further, the magnets are attracted to any nearby iron, creating lateral forces on the magnets, distortions in the magnetic field, inductive heating, and the like. Moreover, such stresses create the risk of quenching the superconducting magnet.
When the magnet is constructed without a ferrous return path, high fringe fields are generated. The fringe fields can be minimized by minimizing the patient gap, i.e., at the expense of patient access and utility. Moreover, the high attractive forces between the pole pieces still require substantial structural elements to keep the pole pieces apart.
The present invention contemplates a new and improved magnetic resonance system which overcomes the above-referenced problems and others.